(1) Field of the Invention
The present invention relates to a process for the preparation of thermoplastic resin sheets, especially thermoplastic resin sheets made from thermoplastic resins having a low electric resistance in the molten state, such as polyamide resin sheets and saponified ethylene/vinyl acetate copolymer (hereinafter referred to as "EVOH") resin sheets. More particularly, the present invention relates to a process for preparing a thermoplastic resin sheet having a smooth surface and an excellent appearance wherein a thermoplastic resin extruded in the form of a sheet from a die of an extruder is pinned firmly to the surface of a quenching roller to rapidly cool the resin sheet by the electrostatic pinning method.
(2) Description of the Related Art
As the conventional process for the preparation of a thermoplastic resin sheet, there can be mentioned a process in which a molten thermoplastic resin sheet melt-extruded from a die is pinned to the surface of a quenching roller to rapidly cool the sheet by the electrostatic pinning method (see, for example, Japanese Examined Patent Publication No. 37-6142).
The volume resistivity of a polyamide resin or EVOH resin in the molten state is in the range of 10.sup.4 to 10.sup.5 .OMEGA.-cm and is much lower than that of, for example, polyethylene terephthalate or polypropylene, and thus, the polyamide resin or EVOH resin in the molten state is highly electroconductive. Accordingly, even if electrostatic charges are applied to a molten polyamide resin or EVOH resin, a large quantity of electro static charges leaks to a quenching roller and the quantity of electrostatic charges per unit area of the sheet becomes small, and a strong electrostatic attracting force cannot be obtained and therefore, a high sheet-manufacturing speed cannot be obtained.
The present inventors have found that when a nylon-6 sheet is prepared by an ordinary electrostatic pinning method, for example, the method disclosed in Japanese Examined Patent Publication No. 37-6142 described above, the sheet-forming speed capable of providing a sheet having a uniform thickness, smooth surface and excellent appearance is limited to about 25 m/min or lower, and if the sheet-forming speed exceeds this critical level, a pinning trouble called "pinning bubble" occurs, whereby firm pinning and rapid cooling cannot be attained, and thus, a uniform sheet cannot be obtained. Accordingly, the preparation of sheets at a speed of about 25 m/min is not advantageous because of a high manufacturing cost.
Various proposals have been made to cope with this problem. For example, there have been proposed a process in which an electrical insulating layer is formed on the surface of a quenching roller composed of a metal to control leaking of electrostatic charges to the quenching roller from a sheet of a thermoplastic resin such as polyethylene terephthalate or polyamide and increase the pinning force (see, for example, Japanese Examined Patent Publication No. 48-14784, Japanese Examined Patent Publication No. 48-29311 and Japanese Unexamined Patent Publication No. 61-95925), and a process in which a large discharge current is applied to a sheet under a corona discharge in the state of a streamer corona from a pinning electrode, whereby the charge quantity is increased to increase the pinning force (see, for example, Japanese Examined Patent Publication No. 59-23270).
In the conventional process in which a corona discharge is carried out in the state of a streamer corona (Japanese Examined Patent Publication No. 59-23270), a large electric current is necessary for effecting the corona discharge in the state of streamer corona, and therefore, a serious problem arises in connection with safety because of the risk of an electric shock. Moreover, a needle electrode is used in this process, and because of the structure of this pinning electrode, sublimates from the molten sheet, such as monomers and oligomers, tend to adhere to and accumulate on the electrode, and to maintain a stable corona discharge, the electrode must be frequently cleaned or exchanged. Accordingly, it is impossible to continue the preparation over a long period. Moreover, the generation of corona is rendered uneven because of contamination of the needle top of the electrode by the deposited and accumulated sublimates, insufficiently pinned portions are formed in the sheet, and the formed sheet is often contaminated with the deposited sublimates falling on the sheet. To avoid this disadvantage, the electrode must be frequently cleaned, and therefore the productivity is inevitably reduced. Furthermore, the needle top of the electrode is unevenly damaged when the electrode is used for a long time, and accordingly, in many needles arranged in a line along the width direction of the sheet, their heights become uneven, even though slightly, and, as a result, corona cannot uniformly be generated in width direction of the sheet and uneven pinning occurs in the sheet, and thus, it is very difficult to maintain the electrodes in good condition.
The conventional process in which an electrical insulating layer is formed on the surface of a quenching roller (see, for example, Japanese Unexamined Patent Publication No. 61-95925) is significant in that the leakage of static charges is minimized in the electrostatic pinning of a thermoplastic sheet having a low volume resistivity. However, even if this process is practically worked, a sheet having a smooth surface and an excellent appearance cannot be always obtained, the sheet-forming speed is not satisfactorily improved, and the process still involves the following problems.
In the first place, when a thermoplastic resin sheet is prepared by the electrostatic pinning method using a quenching roller having an electrical insulating layer as proposed in the above patent publication, the appearance is often degraded by an infinite number of fine circular shaped bubbles having a diameter of about 0.1 to about 0.2 mm, which are formed over the entire surface of the obtained sheet, and this sheet looks hazy and has a frosted glass-like appearance.
It is considered that this appearance defect occurs because an infinite number of air bubbles are included in the interface between the quenching roller and the sheet pinned thereto and are crushed to small bubbles by the pinning force of the sheet. In the case of a mirror-polished metal quenching roller, even if the pinning is unsatisfactory, the appearance defect caused by an infinite number of fine circular bubbles formed over the entire surface of the sheet is not found. Therefore, it is construed that this appearance defect is due to the electrical insulating layer formed on the surface of the quenching roller.
In the second place, according to the kind of the electrical insulating layer or according to the degree of the electrical insulating property or surface roughness even in the same layer material, the pinning property varies, and in an extreme case, an improvement of the sheet-forming speed cannot be expected.
In the third place, when a thermoplastic sheet is prepared by using a quenching roller having an electrical insulating layer, upon separating the cooled sheet from the roller, peeling charges occurs and the charges formed on the surface of the quenching roller result in an unsatisfactory pinning and a generation of sparks, and therefore, a thermoplastic sheet cannot be prepared stably and efficiently.
This third problem will now be described in detail with reference to the accompanying drawings. When a quenched thermoplastic resin sheet is peeled from a quenching roller, as shown in FIG. 8 (prior art), the charging occurs on both sides of the sheet (10) and the surface of a quenching roller (2). Where the sheet (10) is a polyamide resin sheet, because of the difference in the electrification rank between the polyamide resin sheet and the ceramic insulation layer (9), in general, the polyamide resin sheet is positively (+) charged while the ceramic layer (9) is negatively (-) charged. In this case, the charging voltage is sometimes larger than +20 KV in the polyamide resin sheet (10) and larger than -2 KV in the ceramic layer (9). This phenomenon is similarly observed where the sheet is an EVOH resin sheet. Accordingly, the surface portion exposed by peeling of the quenched sheet is negatively (-) charged and negative (-) charges are applied to the surface of the molten resin sheet (10) extruded from the die (1), by the electrode (3), and thus an unsatisfactory pinning occurs due to a repulsion among the negative (-) charges. Furthermore, the negative (-) charges on the surface of the molten resin sheet (10) are summed with the negative (-) charges on the exposed surface of the quenching roller (2), and the voltage between the electrode and the metal substrate of the quenching roller (2) is elevated and sparking often occurs. This repulsion and generation of sparks can be avoided by positively (+) charging the surface of the molten resin sheet (10). In this case, however, it is difficult to generate a uniform corona discharge over the entire width of the electrode (3), but a streamer corona is generated locally and an abnormally large electric current flows locally, resulting in an unsatisfactory pinning.
Therefore, according to the process of Japanese Unexamined Patent Publication No. 61-95925 shown in FIG. 8, the sheet-forming operation can be carried out conveniently only under limited conditions such that charging on peeling does not occur, or even if separation charging on peeling occurs, the charges on the surface of the roller are very small.
As a means for overcoming this disadvantage, there is known a process shown in FIG. 9 (prior art) in which static charges having a reverse polarity to that of the charges on the surface of the molten resin sheet and applied to the exposed surface portion of the quenching roller (2) (see Japanese Examined Patent Publication No. 48-29311). According to this process, however, with an increase of the sheet-forming speed, the extinction of electrostatic charges or charging with a reverse polarity on the surface of the quenching roller (2) becomes uneven and the pinning position of the molten sheet to the quenching roller (2) is slightly changed, the pinning line on the quenching roller is disturbed in several places, namely, the pinning line becomes non-straight, with the result that the thickness of the sheet (10) becomes uneven or several points of unsatisfactory pinning appear over the width direction of the quenching roller, and undesirable streaks extending in the conveying direction readily appear on the sheet due to a linear unsatisfactory pinning. This disadvantage will be overcome by increasing the voltage between the pinning electrode and the quenching roller to enhance the extinction of charges or charging with a reverse polarity. In this case, however, sparks are readily generated, and especially in the case of a ceramic insulation layer (9), holes are formed by these sparks and since a repair of holes is difficult, the insulation layer (9) must be frequently renewed, with the result that the sheet-preparing speed cannot be enhanced.